Bury the Buzzword: Envelope Crystals

This buzzword refers to the appearance of calcium oxalate dihydrate crystals that can be found in the urine of patients with ethylene glycol toxicity, or kidney stones (nephrolithiasis). They look like squares with an “X” in the center, which appear similar to a mail envelope.

The “envelope” appearance is specific to the calcium oxalate dihydrate crystals only. Another type of calcium oxalate crystals exist, termed calcium oxalate monohydrate crystals. These crystals have a needle-shaped appearance, and can also be found in the two above conditions.

It is important to take the clinical presentation into context when the presence of these crystals are encountered, especially in the case of possible ethylene glycol ingestion. Unless there is a plausible history of poisoning, use caution in making this diagnosis because these patients could also very well have a kidney stone!

A picture of these crystals might show up on the USMLE or COMLEX. If they do, you will now be able to make the diagnosis using the question stem to put them in the correct clinical scenario.

Sources:

Preminger, GM, and GC Curhan. The first kidney stone and asymptomatic nephrolithiasis in adults. In: UpToDate, Basow, DS (Ed), UpToDate, Waltham, MA, 2011.

Sivilotti, ML, and JF Winchester. Methanol and ethylene glycol poisoning. In: UpToDate, Basow, DS (Ed), UpToDate, Waltham, MA, 2011.